Thermionic cathode lamp and method of operating the same



A. w. HULL Oct. 3, 1933.

THERMIONIC CA THODE LAMP AND METHOD OF OPERATING THE SAME Filed Oct.'15,

' coni'al'ns neon a Ue m m n m et VP he m s A H Patented Oct. 3, 1933PATENT OFFICE.

TH ERMI ONIC CATHODE LAMP AND METHOD OF OPERATING THE SAME Albert -W.Hull, Schenectady, N. Y., allignor to General Electric Company, acorporation New Iork Application mm 15,1921.

No. ezaa'zs 17 Claims. (01. 176-424) The present invention relates togaseous glow lamps, that is, to lamps in which an electric discharge isconducted for illuminating purposes through a column of suitable gas orvapor, such,

for example, as neon or mercury vapor.

In accordance with my invention I haveproduced a new form of gaseousglow lamp having a higheiliciency, long life and other advantages andwhich isprovided with a thermionic cathode. Glow lamps heretofore havebeen provided with non-thermionic electrodes, that is, electrodes whichare caused to emit electrons by bombardment but which are not operatedduring use at i a temperature of thermal electron emission. As theelectron emissivity of such non-thermionic electrodes is relativelysmall per unit of area, it has been necessary for practical purposes tooperate columnar lamps with currents well below 100 milliamperes. Evenat this relatively low current value. it was necessary to provide such alamp with awkwardly large electrodes.

In such gaseous glow lamps having non-thermionic or -cold electrodesthere is a relatively large voltage drop at the cathode which representsa loss of energy, and which causes disintegration or sputtering of thecathode and blackening of the lamp walls. In the case of neon lamps thisvoltage drop is about 200 volts, or more. In order that this largevoltage drop should not constitute too large a fraction of the totaloperating voltage and thus render the operating emciency of such lampsuncommercially low, it was necessary to construct such lamps foroperation at voltages of several thousand volts. I The intrinsicbrilliancy oi the illumination of such low current lamps beingcorrespondingly low, their usefulness was limited. For example, theycould not be used most eilectively'for display or advertising purposesin places where thegeneral illumination is high, the contrast whichmakes such monochromatic illumination effective being partially lostunder these conditions. g

It was suggested heretofore to substitute a thermionic filamentarycathode in place of the '45 non-thermionic cathodes ingaseous glowlamps,

but the lamps proposed were not suitable for operation with substantialcurrents nor for long periods and'the filamentary cathodeswere subjectto excessive electrical disintegration orsput I have discovered thatthermionic cathode gaseous glow lamps can be constructed to give athousand hour or longer period or operation with currents higher than100 milliamperes (1/10 ampere),ifthecathodefallofpotentialismaintainedbelow a limiting value which I have termed the disintegration voltage.This may be doneby socorrelating tlie electron emissivity of the cathodewith relation to the space or glow current which the lamp is designedtocarry that the fall of potential at the cathode does not exceed thedisintegration voltage. The "disintegration voltage is always greaterthan the ionization voltage of the gas content oif a lamp and in a lamphaving a luminouscolumn of appreciable length is materially less thanthe difference of potential between the electrodes. a

In operation at current values of high luminous efliciency a commerciallength of lamp life has not heretofore been obtained in gaseous 'glowlamps employing thermionic cathodes. In thermionic gaseous glow lampsheretofore proposed the cathode has been a filament. If such a cathodeis to give an adequate electron emission the filament must either be ofsuch length that the potential drop between its terminals is greaterthan the disintegration voltage, or of increased thickness, in whichcase the magnetic field of the heating current interfereswith orprevents an effective electron emission. If more than one filament wereoperated in parallel the discharge would run from one filament -only andcause its rapid destruction.

While my invention in a general sense includes gaseous glow lamps havingcathodes of any form or shape providing their electron emissivity issufllciently high to support a desired operating current without causingthe fallof voltage at the cathode to be greater than the disintegrationvoltage," in the preferred embodiments .of my invention, indirectlyheated non-filamentary cathodes are employed constructed to furnishelectron emission capable of supplying aspace current which the lamp isdesigned to carry at a temperature of inappreciable thermaldisintegration and which operate either with no potential drop betweenextremities or operate with potential drop less than the disintegrationvoltage; Such electrodes, for example, may assume the form of relativelythin walled hollow bodies coated with a thermionically active materialand being maintained at operating temperature by radiation or conductionof heat from a suitable heater. In an attenuated gas, electrodescoatedwith a thermionically active material can be operated at a highertemperature than in a vacuum without loss of the coating material butthe foundation material slowly evaporates. at the most eifectiveoperating temperature. Unlike filamentary electrodes which are heated bythe 11 Iii application of current at their terminals, electrodes such asherein shown and described will continue to function even after thenickel, or other foundation material, has been'dissipated to such extentthat only a skeleton or lace-work remains. Also, unlike lamps containingfilamentary cathodes, no tendency exists in lamps embodying my inventionfor the discharge to concentrate at the negative terminals. Todistinguish such broad area cathodes over filamentary cathodes whichburn out quickly in a glow lamp due to local disintegration, I shallrefer to cathodes having such effective configuration as broadcathodes."

Lamps embodying my invention having an illuminating column of moderatelength and containing a gas or vapor of high luminosity, such as neon ormercury vapor, may be operated on ordinary commercial lighting circuitsof 110 to 115 volts, with a commercially long life, at lighting,efficiencies formerly attainable only in non-thermionic glow deviceswhen operating at voltages of four to five thousand volts. Lampsembodying my invention may be operated at current values materiallyabove l/10 ampere and ordinarily of one to several amperes with highintrinsic brilliancy, and a luminous efllciency of about 15 lumens perwatt. 'They may be constructed in large units giving a higher totalillumination than was possible heretofore. The discharge is diffuse,filling the entire discharge space, but otherwise arc-like in itscharacteristics.

My invention which includes a new method of lamp operation as well as anew device will be explained in greater detail in connection with theaccompanying drawing of which Fig. 1 is a side elevation of a preferredembodimentof my invention containing a permanent gas; Fig. 2 illustratesan operating circuit for alternating current and also a modification inwhich mercury vapor is used; Fig. 3 illustrates a direct currentoperating circuit and also shows in side elevation another modificationin which the enlargement of the envelope about the cathode is absent,and Figs. 4 and 5 are enlarged sectional views of cathode structures.

Referring to Fig. l, the lamp here shown embodying my invention has atubular envelope 1 consisting of glass, silica, or other suitabletransparent material. This envelope has been shown as broken to indicatethat it has a considerable and v'aried length depending in anyparticular case on conditions. For example, for a 110 volt 'lamp,containing neon an envelope 2.5 cm. in

diameter and about 50 to 60 cm. long may be used. The anode 2 carried bya stem 3, which is sealed into the press, 4, is of the usualconstruction, and may consist of nickel, iron, molybdenum, their alloys,or other suitable material. The cathode 5 may be constituted of arelatively thin cylinder of nickel coated with a suitable thermionicallyactive material. I may use, for example, an alkaline-earth compound,such as barium carbonate, preferably with a suitable binder, such as asolution of a cellulose compound. This cathode is heat treated to renderit active. This treatment consists in raising the temperature of thecoated cathode to about 1050 to 1300 C. while the envelope 1 is beingevacuated of gas. In the case of a foundation of nickel coated withbarium, the forming treatment pref erably should be carried close to themelting point of nickel. Evacuation is continued during this heattreatment in order to remove gaseous decomposition products. The bariumcarbonate is decomposed and a compound or alloy of barium and nickel isformed which has a metallic appearance as contrasted with the whiteappearance of the barium carbonate coating. During the operation of thelamp the cathode should be heated to a temperature somewhat below theformation temperature of the coating, say, to a temperature of about1000 C.

' The cathode is provided with an internal heater 6 consisting oftungsten, or other suitable material, receiving current by theconductors 7, 8, which are sealed into the press 9, the conductor 7being joined to one end of the heater 6, andthe conductor 8 being joinedto the closed end of the cylinder 5. The latter in turn is joinedelectrically to the opposite end of the heater. The cathode structure iscarried by a stem 10 fused into the glass press 9. The envelope 1 isexpanded about the cathode into the form of a bulbous cathode chamber11, to avoid overheating. It is charged after thorough evacuation with asuitable gas, such as neon, for example, at a pressure of, say aboutseveral microns to several mm. of mercury. As shown in Fig. 2, a drop ofmercury 12 may be introduced as a source of vapor. The length of thedischarge path should be great enough to cause the fall of potential inthe luminous column to be greater than the combined fall of potential atthe electrodes, and ordinarily should be chosen to cause this-potentialdrop to be several times greater than the combined electrode potentialdrop. A tube having a diameter of about 2.5 cm. containing neon at apressure of about two millimeters of mercury has a voltage drop of aboutone volt per cm. with a current of about three amperes. At lowercurrents the voltage drop is greater as at this pressure the volt-amperecharacteristic is slightly negative. This voltage drop varies inverselywith the tube diameter. The potential drop is substantially constantover a pressure range from about two to five millimeters. The lightintensity varies .substantially as the 0.67 power of the current. As

the voltage drop across the lamp decreases with increase in current, theluminosity is roughly proportional to the wattage consumption. A neonlamp embodying my invention operating with a drop of potential of about120 volts in the lamp at a current of 2.9 amperes, and an energyconsumption at the cathode of about 34 watts, gave a light emission ofabout 5350 lumens, that is, an efficiency of the discharge of aboutfifteen lumens per watt, and an overall efiiciency of 10.7 lumens perwatt, that is, taking into account losses in the stabilizing resistanceand other losses.

A lamp embodying my invention may be constructed for operation eitherwith direct or alternating current. I have illustrated in Fig. 2 a lamphaving two anodes 14, 15, located in the branches 16, 17 of the vitreousenvelope, these anodes being connected to the opposite terminals of thesecondary of a transformer 18. The cathode 19 is connected by theconductor 20 to an intermediate point of the secondary of thetransformer 18. An auxiliary transformer 21 is shown for heating thecathode although a battery or other suitable heating means may be used.The conductor 20 is connected in series with the coil of a contactor 22to an intermediate point of the secondary of the-transformer 18, thatis, to approximately the neutral point.

A high frequency device, such as illustrated at 23 may be employed tofacilitate starting of the discharge, not only of the device shown inFig.

means bodying my invention. Such device and its action in starting iswell known and hence will not be here described. when an operatingcurrent flows through the lamp, the magnet of the contactor 22 isenergized, attracting its armature 24 and opening the-primary circuit ofthe cathode heating transformer 21. The main current, which now passesthrough the cathode heater on its way to the anode maintains the cathodeheated to an operating temperature. The cathode of the device shown inFig. 2, as better shown in Fig. 5, is provided with two heater spirals25, 25, connected in series with one another and also connected at 26 tothe cathode shell 19. These heaters 25, 25 are imbedded in suitablerefractory, insulating material 27, for example.

magnesium oxide, contained within the shell 25.

The exterior of the shell 19 is coated with suitable activatingmaterial. Current is supplied by the conductors 28, 28'. A

Another modification of my invention is shown in Figs. 3 and 4. Thecathode cylinder 29 is hollow and contains a single heater spiral 30,one end of which is connectedto a supply wire 31, the other end beingconnected to the shell 29. The shell 29 in turn is electricallyconnected by the ring-shaped member 32 to the shield 33. The shield 33conserves heat and coating material which otherwise would be lost byradiation and evaporation from the cylinder 29 and thereby materiallyincreases the efficiency and life of the cathode. Contact is made tothis shield by a sealed-in conductor 34. A steadying support 35 isprovided at the side of the cathode opposite the point of connection tothe conductor 34. The exterior of the shell 29 and the interior of theshield 33 are coated with a material of high thermionic emissivity asdescribed above. A spacingand insulating sleeve 36 consisting ofalumina, or the like, preferably is provided between the conductor3l|and the shell 29. Either a fixed gas or a vapor may be introducedinto the envelope 1, Fig. 3, after the gas and moisture content havebeen removed and the cathode has been activated.

The lamp of Fig. 3, which is provided with such a cathode, requires noenlargement about the cathode because M the shielding effect of thecylinder 33. The lamp isshown connected by a circuit 3'1 to a directcurrent source 38 in series with a resistance 39, a hand operated switch40, and the coil 41 of a mercury switch 42. The mercury switch 42 is ina parallel circuit 43 containing a resistance 44 and a switch 45. Whenthe switches 40 and 45 are closed, current flows in the heater circuit43, which thereupon is opened by the mercury switch, causing a highvoltage impulse to be impressed upon the lamp. This action of the coil41 may be repeated a'number of. times until the glow discharge isstarted.- If desired a heating current also may be impressed on theconductors 31, 34 to bring the cathode to an operating temperaturepreliminary to starting,- this current being interrupted'after the lampis started.

When the heater 30 is properly proportioned with respect to the-energysource 38, the external resistance 39 may be omitted. The currentthrough the gas in the lamp flowing through the heater 30 maintains thecathode at operatingtemperature. Although in the preferred forms or myinvention the cathode heater serves also as a resistance for limitingthe space or glowproducing current to or below the limiting value abovewhich the cathode drop of potential'would exceed the disintegrationvoltage and cause excessive sputtering of the cathode by positive ionbombardment, I wish it to be understood that various known means, suchasan external resistance or a constant current source may be used forthe same purpose. f

In U. S. Patent 1,790,153 filed concurrently herewith broad claims aremade for gas-containing electrical discharge devices which are pro-.'

vided with thermionic cathodes constructed and proportioned to provide athermionic electron emission suflicient to support in the absence ofpositive ion bombardment the operating current such devices are designedto carry.

What I claim as new and desire to secure by Letters Patent of the United'States, is':

1. An electric lamp comprising a sealed envelope, a luminosity producing,gas therein at about one millimeter of mercury pressure, and

cooperating electrodes including a broad thermionic cathode which iscapable of supporting an arc discharge in said gas of at least anampere,

said electrodes being separated a distance sufllciently great to causethe totalvoltage drop between the same to be several times theionization voltage of said gas, a source ofcurrent and means forlimiting the value of the space current in said device withrespectto'the electron emission of the cathode to maintain the fall ofvoltage at said cathode less than the cathode disintegration voltage.

2. An electric lamp comprising an elongated, tubular envelope, a chargeof neon gas therein at-a pressure of about one to five millimeters ofmercury, electrodes therefor comprising a broad thermionic cathodehaving an electron emissivity of several amperes at about 1000? C., anelectric heater for said cathode and means for electrically operatingsaid lamp in series with said heater.

3. An electric glow lamp comprising a lighttransmitting envelope, acharge of gastherein which is capable of producing luminosity and havinga pressure within the range of a few microns to a few millimeters ofmercury, electrodes mounted in said envelope, one of said electrodesbeing coated with a material of high emissivity, and a shield spacedclosely about said coated electrode.

' 4. An electric lamp comprising a light transmitting envelope, anelectrically luminous gas therein, and electrodes in said envelopespaced apart such distance that the fall of potential in the luminouscolumn between'said electrodes is greater than the combined potentialfalls at said electrodes, said electrodes including a cathode coatedwith alkaline-earth material of high electron emissivity and having anarea large enough to support without the aid of positive ion bom-'bardment an electrical discharge insaid gas of at least about an ampereat a temperature of inappreciable thermal volatiliz ation, and means forheating said cathode.

5. An electric lamp comprising a light-transmitting envelope, a chargeof neon gas therein at a pressure of about one millimeter of mercury andelectrodes mounted in said envelope at regions spaced apart suchdistance that ina luminous discharge between said electrodes the fall ofpotential in the gas between said electrodes will materiallypreponderate over the fall of potential at said electrodes, one of saidelectrodes being a hollow'metal body coated with alkaline earthmateriaLand an electrical resistance heater for said coated electrode,said coated electrode v to several amperes substantially independently apressure of about several microns to several' millimeters of mercury,and electrodes mounted in said envelope at regions spaced apart suchdistance that in a luminous discharge between said electrodes the fall01- potential in the gas between said electrodes will materiallypreponderate over the fall of potential at said electrodes,

one 01' said electrodes being a hollow body oi nickel coated withalkaline earth material of sumcient area to maintain without positiveion bombardment a current the lamp is designed to carry at a temperatureor about 1090 C. and a heater for said coated electrode.

7. An electric lamp comprising a container, a

. charge 01' luminosity-producing gas therein at a pressure within therange 01' about several microns to several millimeters, electrodescomprising a thermionic cathode and an anode spaced apart a sufllcientdistance to have during operation a voltage therebetween which ismaterially higher than the disintegration voltage for said gas, saidcathode providing at a temperature 01' inappreciable thermaldisintegration a thermionic electron emission suilicient to supplywithout the aid of positive ion bombardment substantially the entireoperating current which said lamp is designed to carry.

8. An electric lamp comprising an envelope, a luminosity-producing gastherein, and electrodes therein spaced apart a suilicient distance topermit oi the formation 01' a luminous positive column therebetweenhaving a fall of potential materially higher than the disintegrationvoltage for said gas, one of said electrodes being a ther-' mioniccathode which is capable of supporting an arc discharge of a currentvalue of about one of positive ion bombardment of said cathode, and witha voltage drop at said cathode below the disintegration voltage for saidgas.

9. An electric lamp comprising a transparent envelope, a charge ofluminosity-producing gas therein at a pressure within the range of aboutseveral microns to several millimeters, electrodes comprising athermionic cathode and an anode having a fall of voltage therebetweenduring operation which .is materially higher than the disintegrationvoltage for, said gas, said cathode being constructed and proportionedto provide at a temperature of inappreciable thermal disintegration athermionic electron emission to supply substantially the entireoperating current which said lamp is designed to carry withoutthe aid ofposiiive ion bombardment oi. said cathode, and means for limiting theoperating current not to exceed said value.

10. An electric lamp comprising a sealed envelope, a gaseous fillingtherefor, electrodes therein one of which comprises a metal shell, amaterial of high electron emissivity coating said body, means forheating said body to a temperature of thermionic emission, saidelectrode being capable at a temperature of inappreciable disintegrationoi emitting an electron current 01' at least about an ampere, and ashield spaced closely about said body whereby radiation oi heat and lossoi coating material are materially reduced.

11. An electric lamp comprising a sealed envelope, an attenuated gastherein, a thermionic i,m,iss

cathode thereinwhlchis capable at operating oi emitting an electroncurrent of at least about an ampere, comprising a. metal shell, a heatertherefor, a material oi. high electron emlssivlty coating said shell,and a metal 1 shield closely spaced around said shell, and an anodecooperating with said cathode.

12. An electric lamp comprising a sealed envelope, a gaseous illlingtherefor at a pressure of about one millimeter oi mercury, electrodestherein one oi which comprises a hollow metal .body, an electric heaterfor said hollow electrode and means for electrically operating said lampin series with said heater.

13. An electric lamp comprising an elongated tubular enevlope, a gastherein at a pressure within a range of about several microns to severalmillimeters of mercury, electrodes therein one of which comprises ahollow metal body, a coating thereon or thermionically active material,an electric heater enclosed by said hollow electrode, and means forelectrically operating saidlamp in series with said heater.

14. The method of operating a positive column electric lamp containing ahighly attenuated gas, a thermionic cathode coated with a material ofhigh electron emissivity and a -cooperating anode which consists inheating said cathode to a temperature oi inappreciable thermalevaporation and conducting between said electrodes a current having avalue not in excess of the limiting value at which the fall of potentialat the cathode rises above the disintegration voltage for said gas.

15. The method of operating at voltages of at least about 110 volts apositive column electric lamp containing a gas at a pressure within therange of about several microns to several millimeters of mercury, andbeing provided with a thermionic cathode and an anode, which consists incausing said cathode to emit electrons capable oisupporting a designedcurrent at a temperature at which disintegration 01' said .cathode isinappreciable and limiting the current between said electrodes to thedesigned value.

16. The method of operating'a positive column electric lamp containing agas at a pressure of about a millimeter of mercury, a thermionic cathodeand an anode spaced apart at least about '50 centimeters and withimpressed voltages materially higher than the cathodic disintegrationvoltage for said gas which consists in causing said cathode to emit bythermionic emission a current of about one to several amperes at atemperature at which thermal disintegration of said cathode isinappreciable and operating between said electrodes 9. current not inexcess of the limiting value at which the fall of potential at thecathode rises above the disintegration voltage for said gas.

17. An electric lamp comprising a sealed envelope, aluminosity-producing gas therein, electrodes therein spaced apart asuilicient distance to permit oi the formation oi a luminous positivecolumn discharge therebetween, and including a

